Introduce a new scheduler module with fiber-based cooperative multitasking, including native bindings for resume operations and a Wren class for scheduling callables. Refactor the existing timer module and CLI code to use a centralized built-in module registry in modules.c, replacing direct includes of timer.h and timer.wren.inc. Update Xcode project and Makefile to compile the new scheduler.c, modules.c, and their generated .wren.inc sources.
Replace the manually embedded C string constants for builtin Wren modules with
a new build system that automatically generates `.wren.inc` header files from
`.wren` source files. The old `script/generate_builtins.py` is removed and
replaced by `script/wren_to_c_string.py`, which produces standalone include
files with a `PREAMBLE` containing the module name and source path. The Makefile
gains pattern rules to rebuild these `.wren.inc` files whenever the
corresponding `.wren` source changes, and the `builtin` phony target is removed
since the generation is now automatic. The `wren.mk` wildcard lists are updated
to include the new `.wren.inc` files as dependencies. The `timer.c` and
`wren_core.c` source files now `#include` their respective `.wren.inc` headers
instead of defining the string literals inline, and the `timer.wren` trailing
newline is removed.
